The devices: how pumps move fluid and what they sense
Name the pump type in front of you, explain how its drive moves fluid, and say what each of its sensors measures and what it cannot detect.
One job, several machines
Every infusion device does one job: it moves a prescribed volume of liquid into a patient at a prescribed rate, and it stops and alarms when something goes wrong. What differs between types is where the liquid comes from, how it is pushed, and how small and how steady the flow has to be. Before you open any pump, decide which kind it is, because that tells you which mechanism, which disposable and which sensors you are dealing with.
In a hospital you will meet five families. They share electronics, batteries and alarms, but their mechanics and their typical faults are different.
- Volumetric (large-volume) pumps: draw fluid from a bag or bottle through a dedicated administration set and push it with a peristaltic or cassette mechanism. Used for fluids, antibiotics, nutrition and many drug infusions.
- Syringe pumps (syringe drivers): push the plunger of a syringe with a lead screw. Used for small volumes and low, steady rates: sedation, inotropes, insulin, neonatal infusions.
- PCA pumps (patient-controlled analgesia): usually a syringe or cassette pump with a patient handset, a lock-out time and a lockable drug reservoir, so the patient can give himself a limited dose of analgesic.
- Enteral feeding pumps: deliver feed into the gut, not into a vein. Lower accuracy demands, but thick fluids and sugar residue are hard on the mechanism, and connectors are made deliberately incompatible with intravenous ones.
- Ambulatory pumps: small, battery-powered pumps worn by the patient for home chemotherapy, pain relief or long infusions. Small mechanisms, heavy dependence on the battery and on the patient's handling.
Drive mechanisms: peristaltic, cassette, lead screw
A linear peristaltic pump presses a row of fingers in sequence against a section of the set's tubing, which lies in a channel behind the door. The fingers form a wave that pushes trapped fluid forward; at least one finger is always closing the tube, so the pump also acts as a valve. A camshaft driven by a stepper motor moves the fingers. Rotary peristaltic pumps do the same job with rollers on a rotor, common in enteral pumps. Accuracy depends on the tubing: its inside diameter, wall thickness and elasticity decide how much fluid one wave moves. That is why a volumetric pump is only accurate with the set the manufacturer validated, and why old, flattened tubing under-delivers.
A cassette pump uses a rigid plastic chamber in the disposable, which a piston or plunger in the pump fills and empties through small valves. The volume per stroke is fixed by the cassette, not by soft tubing, so the cassette and its seating matter most.
A syringe pump holds the syringe barrel in a cradle with a clamp and pushes the plunger with a carriage (also called the pusher or driving head). The carriage rides on a lead screw turned by a stepper motor through a gearbox; a half-nut engages the screw and can be released with a lever so the carriage can be moved by hand. Delivered volume depends on how far the carriage moves and on the inside diameter of the syringe, so the pump must know exactly which syringe is loaded. At very low rates, the friction of the rubber plunger and the play in the mechanism decide how quickly flow starts and how smooth it is.
Whatever the mechanism, the motor is commanded in steps, and the pump calculates volume from steps. It does not measure the fluid that actually leaves it. Keep that in mind: a pump can display a perfect rate while delivering the wrong volume because the set, the syringe or the mechanism is not what the software assumes.
Sensors: what they measure and what they miss
Pumps protect the patient with a small set of sensors. You must know what each one really measures, because many 'false alarms' are the sensor working correctly on a physical problem the user does not see.
Two limits of these sensors matter. First, they detect conditions near the pump, not at the patient: a disconnected line or an infiltrated cannula may cause little or no pressure change. Second, a sensor that drifts or is dirty can fail silently in the unsafe direction, which is why the performance tests in lesson 3 check that each alarm really triggers.
- Downstream (patient-side) pressure sensor: a force or strain sensor pressing on the tubing or cassette membrane, or a force sensor in the syringe carriage. It sees pressure building when the line is blocked or kinked. The occlusion alarm level is often selectable within limits set by the manufacturer.
- Upstream pressure sensor (volumetric pumps): sees the tube collapsing when the bag is empty, the roller clamp is closed or the spike is blocked.
- Air-in-line detector: usually an ultrasonic transmitter and receiver across the tube below the mechanism. Liquid carries ultrasound well, air does not. Some pumps count single bubbles and also accumulate small bubbles over time.
- Door and set-loading sensors: confirm the door is closed and the set or cassette is seated, often with a feature on the set that also engages the anti-free-flow clamp.
- Syringe size detection: a sensor on the barrel clamp, typically a position sensor such as a linear potentiometer (designs differ by model), measures the barrel diameter; the pump compares it with the syringe brand and size selected or recognised.
- Plunger detection and plunger clamp sensors: confirm the plunger flange is held by the carriage claws, so the syringe cannot empty by itself under gravity or negative pressure.
- Carriage position sensor: tells the pump how far the plunger has travelled and therefore when the syringe is near empty.
- Drop sensor (on some volumetric pumps and controllers): an optical sensor on the drip chamber that confirms flow and detects an empty container.
Drug libraries and dose error reduction at concept level
Most current pumps can carry a drug library, a data set prepared by the hospital pharmacy that lists drugs, concentrations and limits for each clinical area. A system that uses it is called dose error reduction software (DERS) or, in marketing, a smart pump. When the nurse picks a drug from the library, the pump checks the entered rate or dose against soft limits (which the user can override with confirmation) and hard limits (which cannot be overridden).
For you as an engineer, three points matter now. The library is clinical content owned by the pharmacy and the clinical teams, not by engineering. It is loaded and updated with the manufacturer's software, often over a network or a docking station. And it only protects the patient if staff actually select the drug instead of programming a 'basic infusion' without limits. Lesson 5 covers your role in detail.
Power, nurse call and docking
Pumps run on mains through an internal or external power supply and carry a rechargeable battery so they can travel with the patient and survive power cuts. Older pumps used nickel-metal hydride packs, most current ones use lithium-ion; both age, and the pump's runtime estimate becomes unreliable as the pack wears. Many battery packs carry their own monitoring electronics, which the pump reads. Lesson 4 is about batteries.
Many pumps can be connected to the hospital nurse-call system, so an alarm also reaches the nurses' station, and can be mounted in a docking station or rack. The rack can supply power, combine several pumps on one pole, pass data to the network and report alarms centrally. A failed dock contact, a bent pin or a missing nurse-call cable produces complaints that look like pump faults but sit in the rack or the wall.
Your scope at Level 1
At this level you inspect, clean, test, replace user-replaceable and service-replaceable parts as the manufacturer allows, and document. Mechanism adjustments and sensor calibrations (syringe size, pressure, air detector, carriage position) are done only by the manufacturer's procedure, with the tools and software it specifies, by someone trained to do them. You never alter sensor thresholds or protection to make an alarm go away, and you do not use service passwords or software you are not authorised to hold. General electrical safety, electronics and fault-finding method are covered in the esafety, electronics and fault-finding-method courses; this course builds on them.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Peristaltic fingers and camshaft | Push fluid along the set tubing in a wave | Worn cam or fingers, sticky residue, under-delivery |
| 02 | Door, latch and anti-free-flow clamp | Hold the set; close the line when the set is removed | Broken latch, worn clamp, door sensor faults |
| 03 | Pressure sensors (up and downstream) | Detect occlusion and empty container | Drift, contamination, false or missing occlusion alarms |
| 04 | Ultrasonic air-in-line detector | Detect air in the tube below the mechanism | Dirty or scratched faces, nuisance or missed air alarms |
| 05 | Lead screw, half-nut and gearbox (syringe) | Turn motor steps into plunger travel | Wear, dirt, slipping half-nut, uneven low-rate flow |
| 06 | Carriage with plunger claws and force sensor | Push and hold the plunger; sense occlusion force | Lost claws or springs, broken flange sensor, false occlusion |
| 07 | Barrel clamp and syringe size sensor | Hold the syringe and measure its diameter | Drift, syringe not recognised or wrong size |
| 08 | Keypad, display and controller board | User interface and control software | Stuck membrane keys, broken ribbon cable, fluid damage |
| 09 | Battery pack with monitoring circuit | Run the pump off mains; report charge state | Aged cells, wrong runtime estimate, pack not recognised |
| 10 | Power entry, nurse call, dock contacts | Mains, alarm relay and rack connections | Bent pins, dirty contacts, damaged cords |
A syringe pump on an anaesthesia workstation that 'did not see' the syringe
A syringe pump used with an anaesthesia workstation stopped recognising one common syringe size, while identical pumps in the theatre recognised the same syringes without trouble. Engineers first suspected the linear potentiometer on the barrel clamp and wear on the carriage parts, and checked them. Neither was clearly damaged. The pump simply measured the barrel diameter slightly differently from what its stored table expected. Running the syringe size calibration in the manufacturer's procedure, using ordinary hospital syringes of the sizes the procedure calls for, brought recognition back. The pump was then checked with each approved syringe size before it was returned.
When one pump in a fleet rejects a syringe others accept, think of sensor calibration before parts, and do the calibration only by the manufacturer's procedure, then verify with every approved syringe size.
Syringe module that could not hold the plunger
A syringe module built into an intensive care device refused to start properly with a locally made syringe and behaved as if no plunger was engaged. A colleague's first question was whether the small claws that press the plunger flange against the pusher were still there, because they are often lost when the pusher is cleaned or forced. His advice was to check whether the module ran when the plunger was held against the pusher by hand, and otherwise to open the pusher and check the plunger sensor flag and its optical switch. The final cause was not confirmed in the discussion, but the missing claws were the leading suspect.
A plunger detection fault often starts with a missing claw or spring on the carriage. Look at the small mechanical parts before suspecting electronics, and never run a syringe that is not clamped.
The infusion that never quite finished
On a syringe pump, the near-end warning appeared a few minutes before the end, but the infusion never reached its end and the syringe was not driven home. Colleagues pointed at two explanations. Either the syringe was not on the model's approved list, so the pump's idea of the barrel and of the plunger end position did not match the syringe, or the pump had switched to its keep-vein-open mode, which the display should show. Their advice was to compare the syringe with the list in the pump's documentation and check the display and settings. The outcome was not reported, so both are probable explanations, not a confirmed cause.
End-of-infusion and near-empty detection depend on syringe geometry. A syringe that is not on the approved list can make the pump misjudge the end, so check the syringe list before you look for a fault.
On-site checklist
- Identify the pump family before you open it: volumetric, syringe, PCA, enteral or ambulatory.
- Find the manufacturer's list of approved sets and syringes for that model.
- Know which sensors the model has and where they sit.
- Ask for the disposable that was used when the complaint happened.
- Check the event log before you change anything.
- Leave calibrations and mechanism adjustments to the manufacturer's procedure.